Optical signal degradation occurs in optical communication systems, owing to factors such as amplified spontaneous emission (ASE) noise. This degradation is monitored and compensated for to improve system performance. For example, one approach involves providing real-time feedback of signal parameters (such as optical signal-to-noise ratio (OSNR)) to optical amplifiers in an effort to reduce the contributions of these optical amplifiers to the signal corruption [1].
However, in systems where bit-rates approach or exceed 40 Gb/s contemporary electronic monitoring techniques are limited in their response to the rapidly varying signal envelopes. Several all-optical noise-monitoring schemes have been suggested that are not restricted in this way, employing polarisation-nulling [2], [3], non-linear power transfer functions [4], [5], electrical carrier-to-noise monitoring [6] or semiconductor optical amplifiers [7]. These schemes make direct measurements of in-band noise which, unlike spectral techniques that interpolate out-of-band noise levels into the signal band, are not vulnerable to errors arising from the effects of routing and filtering.
However, such all-optical OSNR monitoring techniques can be limited in their sensitivity to changes in OSNR at levels greater than 20 dB [4], [5], and it is desirable to detect changes in OSNR in the region of 30 dB or higher so compensation can be made before noise corruption becomes significant. Furthermore these devices are often highly sensitive to variations in other signal parameters including chromatic dispersion [4],[5] and polarisation mode dispersion (PMD) [2],[4].